Method for shot peening a gas carburised steel

ABSTRACT

The present invention is to provide a method for shot peening for producing a high compressive residual stress in a gas carburized steel that has a soft layer. In this method, a depth where the maximum compressive residual stress is generated is estimated and the hardness on the surface or near the surface is not used. The depth where the maximum compressive residual stress is generated is estimated by multiplying the depth where the maximum stress is generated under contact stresses caused by the collision of shot media by the constant K. A processed steel that comprises a gas carburized steel and that has a hardness at that depth that exceeds 750 HV is used. Shot media that have a hardness that is greater than that of the processed steels at that depth by 50 HV or more are shot onto the processed steels to produce a high compressive residual stress in the processed steels.

TECHNICAL FIELD

The present invention relates to a method for shot peening. Specifically, it relates to a method for shot-peening a gas carburized steel.

BACKGROUND ART

Conventionally, shot peening has been known to produce compressive residual stresses and to increase hardness to improve the fatigue strength of gears (see authored by the Society of Shot Peening Technology of Japan; Fatigue of Metals and Shot Peening; published by Gendai Kogaku-sha; 2004). Further, to achieve high fatigue strength aimed at reducing the weight of parts, a method for increasing compressive residual stresses produced by shot peening has been known (see Kazuyoshi Ogawa and Takashi Asano; Theoretical Prediction of Residual Stress Produced by Shot Peening for Hardened Steel; Transactions of JSSR, No. 48 (2003) pp. 31-38).

However, in these studies only the mean values of the hardness on the surface of, or near the surface of, the material are focused on. Thus the methods by these studies are difficult to be applied to a gas carburized steel that has a soft layer on the surface, since residual stresses are affected by plastic deformations caused by the collision of shot media and the mechanical properties of the processed steels that relate to the suppression of the plastic deformation.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a method for shot peening for producing high compressive residual stresses in a gas carburized steel that has a soft layer on the surface. In this method, a depth where the maximum compressive residual stress is generated is estimated, and no data on the hardness on the surface or near the surface is used.

The method for shot peening of the first aspect of the present invention is to produce a compressive residual stress in a processed steel by peening shot media onto the processed steel. The processed steel comprises a gas carburized steel having a hardness of 750 HV or higher at the depth z, where the maximum compressive residual stress is generated. The depth z is estimated by using Equations (1) to (4) below. The shot media have a hardness that is above that of the processed steel by 50 HV or more.

$\begin{matrix} {\alpha = {\frac{1}{8E^{*\frac{3}{5}}}\left( \frac{5\pi}{4} \right)^{\frac{s}{5}}\rho^{\frac{3}{5}}D^{3}V^{\frac{6}{5}}}} & (1) \\ {\frac{1}{E^{*}} = {\frac{1 - v_{1}^{2}}{E_{1}} + \frac{1 - v_{2}^{2}}{E_{2}}}} & (2) \\ {z = {0.48K\; \alpha}} & (3) \\ {K = 1.25} & (4) \end{matrix}$

where

α: contact radius by shot media (m),

p: specific gravity of shot media (kg/m³),

D: diameter of shot media (m),

V: shot speed(m/s),

E*: equivalent elastic modulus

E₁: Young's modulus of the processed steel(Pa),

v₁: Poisson's ratio of the processed steel,

E₂: Young's modulus of shot media (Pa),

v₂: Poisson's ratio of shot media,

K: constant, and

z: depth where the maximum compressive stress is generated (m).

The method for the shot peening of the second aspect of the present invention is characterized in that the diameters of shot media are in the range from 0.2 mm to 1.0 mm in the method for the shot peening of the first aspect.

By the method for the shot peening of the first aspect of the present invention, the position where the maximum compressive residual stress is generated is estimated by multiplying the depth by a constant. The depth is determined as the depth where the maximum stress is generated by the collision of shot media. By shot-peening a gas carburized steel a maximum compressive residual stress that is equal to, or more than, 1,600 MPa, is produced. At that depth the gas carburized steel has a hardness that is equal to, or more than, 750 HV. The shot media have a hardness that is above that of the processed steel by 50 HV or more. Thus the processed steel is processed to have a high fatigue strength. That is, by estimating the depth z, where the maximum compressive stress is generated, from Equations (1) to (4), it is possible to produce a high compressive residual stress in a gas carburized steel that has a soft layer.

By the method for the shot peening of the second aspect of the present invention, since the diameters of shot media are in the range from 0.2 to 1.0 mm, the maximum compressive residual stress can be securely produced in the processed steel.

The basic Japanese patent application, No. 2010-176681, filed Aug. 5, 2010, is hereby incorporated by reference in its entirety in the present application.

The present invention will become more fully understood from the detailed description given below. However, the detailed description and the specific embodiment are illustrations of desired embodiments of the present invention, and are described only for an explanation. Various possible changes and modifications will be apparent to those of ordinary skill in the art on the basis of the detailed description.

The applicant has no intention to dedicate to the public any disclosed embodiment. Among the disclosed changes and modifications, those which may not literally fall within the scope of the present claims constitute, therefore, a part of the present invention in the sense of the doctrine of equivalents. The use of the articles “a,” “an,” and “the” and similar referents in the specification and claims are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by the context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention, and so does not limit the scope of the invention, unless otherwise claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the distribution of the hardness of the processed steels that were used in the embodiments of the present invention.

FIG. 2 is a table showing the conditions and the results of the shot peening that were used in the embodiments of the present invention.

FIG. 3 is a graph showing the relationship between the estimated values and measured values where the maximum compressive residual stress is generated.

FIG. 4 is a graph showing the relationship between the differences between the hardness of shot media from the hardness at the depth where the maximum residual stresses is generated and the maximum compressive residual stresses.

BEST MODE FOR CARRYING OUT THE INVENTION

Below, the embodiments of the present invention are described with reference to the drawings.

FIG. 1 is a graph showing the distribution of the hardness of the processed steels that were used in the embodiments. The abscissa denotes depths (micrometer) from the surface of the steel, and the ordinate denotes the Vickers hardness. A gas carburized steel is used for the processed steels. In the drawing, “TP.A” and “TP.B” denote the steels which have been tempered at 180 degree C. and 180 degree C., respectively.

FIG. 2 is a table showing the conditions and the results of shot peening that were used in the embodiments. A compressive-air shot peening system was used. Shot media that have a hardness of 700 HV to 1,000 HV and diameters (the mean diameters) of 0.2 to 1.0 mm were used.

The

“Maximum σ_(R)”

in the table denotes the maximum compressive residual stresses in the processed steels. The compressive residual stresses were measured by using a micro-stress analyzer that is available from Rigaku Corporation (X-ray tube: Cr-Kα; diffractive surface: (220); stress constant: −318 MPa/deg; Bragg angle of the strain-free 2θ: 156.4°).

The “Depth of Peak” in the table denotes the depth from the surface where the maximum compressive residual stress is generated. The “Hardness at Peak” denotes the hardness at the “Depth of Peak,” i.e., the Vickers hardness of the processed steels at the depth where the maximum compressive residual stress is generated. The “Relative Hardness” denotes the differences between the hardness of shot media and that of the processed steels, specifically the value that is calculated by subtracting the hardness at peak of the processed steels from the hardness of shot media. As shown in the table, if the relative hardness is 50 HV or more, the maximum compressive residual stress of −1,600 MPa or more can be obtained. The maximum compressive residual stress of −1,600 MPa is a typical value that is required for gear materials.

FIG. 3 is a graph showing the estimated values and measured values of the depths where the maximum compressive residual stress is generated. The estimated values are calculated by using the following Equations (1) to (4).

$\begin{matrix} {\alpha = {\frac{1}{8E^{*\frac{3}{5}}}\left( \frac{5\pi}{4} \right)^{\frac{s}{5}}\rho^{\frac{3}{5}}D^{3}V^{\frac{6}{5}}}} & (1) \\ {\frac{1}{E^{*}} = {\frac{1 - v_{1}^{2}}{E_{1}} + \frac{1 - v_{2}^{2}}{E_{2}}}} & (2) \\ {z = {0.48K\; \alpha}} & (3) \\ {K = 1.25} & (4) \end{matrix}$

where

α: contact radius by shot media (m),

p: specific gravity of shot media (kg/m³),

D: diameter of shot media (m),

V: shot speed(m/s),

E*: equivalent elastic modulus

E₁: Young's modulus of the processed steel(Pa),

v₁: Poisson's ratio of the processed steel,

E₂: Young's modulus of shot media (Pa),

v₂: Poisson's ratio of shot media,

K: constant, and

z: depth where the maximum compressive stress is generated (m).

As shown in FIG. 3, the estimated values where the maximum compressive residual stress is generated are generally coincident with the measured values. That means that the depths where the maximum compressive residual stress is generated can be estimated by multiplying the depth where the maximum stress is generated under contact stresses caused by the collision of shot media, by the constant K.

FIG. 4 is a graph showing the relationship between the differences between the hardness of shot media and the hardness at the depth where the maximum compressive residual stress is generated. Specifically, it shows the values (the relative hardness) that are calculated by subtracting the hardness of the processed steels at the peak depth from the hardness of shot media on the abscissa and the maximum compressive residual stresses (MPa) of the processed steels on the ordinate.

As shown in FIG. 4, if the value that is calculated by subtracting the hardness of the processed steels at the peak depth from the hardness of shot media is less than 50 HV, the maximum compressive residual stress does not reach −1,600 MPa. This is because shot media are subject to plastic deformation when they are shot onto the processed steel. Thus energy is insufficiently transmitted from shot media to the processed steel.

On the contrary, if the value that is calculated by subtracting the hardness of the processed steels at the peak depth from the hardness of shot media is 50 HV or more, the maximum compressive residual stress exceeds −1,600 MPa. Since the maximum compressive residual stress is generally expressed as a minus value, that means that the absolute value exceeds 1,600 MPa. This is because shot media are seldom subject to plastic deformation when they are shot onto the processed steel. Thus sufficient energy is transmitted from shot media to the processed steel.

Further, as shown in FIG. 4, if the hardness of the processed steel at the depth where the maximum compressive residual stress is generated is less than 750 HV, the maximum compressive residual stress does not reach −1,600 MPa even when the value that is calculated by subtracting the hardness of the processed steels at the peak depth from the hardness of shot media is 50 HV or more. The maximum compressive residual stress to be produced in a steel is known as being limited by the yield strength of the steel. The yield strength is proportional to the hardness. Thus, unless the hardness of the processed steel at the depth where the maximum compressive residual stress is generated is 750 HV or more, the yield strength that is required to produce the maximum compressive residual strength of −1,600 MPa cannot be ensured.

The threshold for the difference in the hardness of shot media and the processed steels, i.e., 50 HV, is determined as follows. As shown in FIG. 4, the maximum compressive residual stresses are shown in relation to the values that are calculated by subtracting the hardness of the processed steels at the “Depth of Peak” from the hardness of shot media. An estimated curve is drawn by the least square method. Based on the curve the threshold is determined. The threshold for the hardness at the depth where the maximum compressive residual stress is generated, i.e., 750 HV, is determined as follows. The maximum compressive residual stresses are shown in relation to the hardness of the processed steels. An estimated curve is drawn by the least square method. Based on the curve the threshold is determined.

As discussed above, in the embodiments of the present invention processed steels are used that have a hardness that is greater than 750 HV at the depth z, where the maximum compressive residual stress is generated. The depth z is estimated from Equations (1) to (4). The shot media that have the hardness that is greater than that of the processed steels at the depth z by 50 HV or more are shot onto the processed steels. In these ways a compressive residual stress is produced in the processed steels. In other words, the depth where the maximum compressive residual stress is generated is estimated by multiplying the depth where the maximum stress is generated under contact stresses caused by the collision of shot media by the constant K. The processed steels that have a hardness at that depth that exceeds 750 HV are used. The shot media that have a hardness that is greater than that of the processed steels at the estimated depth by 50 HV or more are shot onto the processed steels. As a result, a maximum compressive residual stress that is 1,600 MPa or more can be produced in the processed steels. Thus the processed steels can be improved in fatigue strength. That is, by estimating the depth z, where the maximum compressive residual stress is generated, from Equations (1) to (4), a high compressive residual stress can be produced in a gas carburized steel that has a soft layer.

Further, by having the diameters of shot media being in the range from 0.2 mm to 1.0 mm, a maximum compressive residual stress that is 1,600 MPa or more can be securely produced in the processed steels.

In this invention, any shot media can be used. However, shot media made of steels, etc., are preferable. 

1. A method for shot peening, wherein a processed steel comprises a gas carburized steel that has hardness at 750 HV or more at a depth z, where a maximum compressive residual stress is generated, the depth z being estimated by using Equations (1) to (4), and wherein shot media that have a hardness that is greater than the hardness of the processed steel by 50 HV or more are shot onto the processed steel to produce a compressive residual stress in the processed steel: $\begin{matrix} {\alpha = {\frac{1}{8E^{*\frac{3}{5}}}\left( \frac{5\pi}{4} \right)^{\frac{s}{5}}\rho^{\frac{3}{5}}D^{3}V^{\frac{6}{5}}}} & (1) \\ {\frac{1}{E^{*}} = {\frac{1 - v_{1}^{2}}{E_{1}} + \frac{1 - v_{2}^{2}}{E_{2}}}} & (2) \\ {z = {0.48K\; \alpha}} & (3) \\ {K = 1.25} & (4) \end{matrix}$ where α: contact radius by shot media (m), p: specific gravity of shot media (kg/m³), D: diameter of shot media (m), V: shot speed(m/s), E*: equivalent elastic modulus E₁: Young's modulus of the processed steel(Pa), v₁: Poisson's ratio of the processed steel, E₂: Young's modulus of shot media (Pa), v₂: Poisson's ratio of shot media, K: constant, and z: depth where the maximum compressive stress is generated (m).
 2. The method for shot peening of claim 1, wherein diameters of the shot media are in a range from 0.2 mm to 1.0 mm. 